Custom ioctl Commands
We have seen that the ioctl system call is
implemented for sockets; SIOCSIFADDR
and
SIOCSIFMAP
are examples of ``socket
ioctls.'' Now let’s see how the third
argument of the system call is used by networking code.
When the ioctl system call is invoked on a socket, the
command number is one of the symbols defined in
<linux/sockios.h>
, and the function sock_ioctl
directly invokes a protocol-specific function (where ``protocol'' refers
to the main network protocol being used; for example, IP or AppleTalk).
Any ioctl command that is not recognized by the protocol
layer is passed to the device layer. These device-related ioctl
commands accept a third argument from user space,
a struct ifreq *
; this structure
is defined in <linux/if.h>
. The SIOCSIFADDR
and
SIOCSIFMAP
commands actually work on the ifreq
structure.
The extra argument to SIOCSIFMAP
, although defined as ifmap
,
is just a field of ifreq
.
In addition to using the standardized calls,
each interface can define its own ioctl commands. The plip
interface, for example, allows the interface to modify its internal
timeout values via ioctl. The ioctl implementation for
sockets recognizes 16 commands as private to the interface:
SIOCDEVPRIVATE
through SIOCDEVPRIVATE+15
.
When one of these commands is recognized, dev->do_ioctl
is called in the relevant interface driver. The function
receives the same struct ifreq *
pointer that
the general-purpose ioctl function uses:
int (*do_ioctl)(struct device *dev, struct ifreq ...
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